Throughout this application various publications are referred to by partial citations within parenthesis. Full citations for these publications may be found at the end of the specification immediately preceding the claims. The disclosures of these publications, in their entireties, are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
An essential property of synaptic transmission is the rapid termination of action following neurotransmitter release. For many neurotransmitters including catecholamines, serotonin, and certain amino acids (e.g., gamma-aminobutyric acid (GABA), glutamate, and glycine), rapid termination of synaptic action is achieved by the uptake of the transmitter into the presynaptic terminal and surrounding glial cells (Bennett et al., 1974; Horn, 1990; Kanner and Schuldiner, 1987). Inhibition or stimulation of neurotransmitter uptake provides a means for modulating the strength of the synaptic action by regulating the available levels of endogenous transmitters. The development of selective inhibitors may therefore represent a novel therapeutic approach to the treatment of neurological disorders.
The amino acid glycine is an important neurotransmitter in the vertebrate central nervous system, where it serves two distinct functions. First, glycine is a classical inhibitory neurotransmitter with a well established role in the spinal cord, brainstem, and retina (Aprison, 1990; Daly, 1990; Cortes and Palacios, 1990). The inhibitory effects of glycine are mediated by the glycine receptor, a ligand-gated chloride channel which is activated by glycine and competitively antagonized by strychnine (Grenningloh et al., 1987). Blockade of glycinergic transmission by strychnine causes seizures in animals and humans. Thus, agents which enhance the inhibitory role of glycine in the CNS may ameliorate the symptoms of epilepsy or other neurological disorders associated with excessive neural and/or musculoskeletal activity. This hypothesis is supported by the finding that defects in the glycine receptor underlie the hereditary myoclonus observed in certain mutant strains of mice (Becker, 1990) and calves (Gundlach, 1990).
In addition to its inhibitory role, glycine also modulates excitatory neurotransmission by potentiating the action of glutamate at NMDA receptors, both in hippocampus and elsewhere (Johnson and Ascher, 1987; for review, see Fletcher et al., 1990). The glycine regulatory site on the NMDA receptor is distinct from the strychnine-sensitive glycine receptor (Fletcher et al., 1990). The NMDA class of glutamate receptors is known to play a critical role in long-term potentiation, a cellular model of learning (Collingridge and Bliss, 1987). Recent evidence suggests that activation of the glycine regulatory site on the NMDA receptor may enhance cognitive function (Handelmann et al., 1989).
The molecular properties of glycine transport, particularly in relation to the dual role of glycine in the nervous system, have not previously been studied. Elucidation of the molecular structure of the synaptic glycine transporter is an important step in understanding glycinergic transmission and modulation. In particular, we were interested in exploring whether separate transporter mRNAs encode the uptake proteins that regulate inhibitory transmission and those that modulate glutamatergic transmission or whether one transporter mediates both functions.